This invention relates to lustrous SnO.sub.2 -coated mica flakes.
Lustrous pigments based on mica flakes coated with metal oxides are known and described in German patent specifications Nos. 1,467,468; 1,959,998; 2,009,566 and in published specification No. 2,214,545. However, under many conditions of use, the light stability of these pigments is less than satisfactory.
The coating of mica with SnO.sub.2 is disclosed in German patent specification No. 1,467,468, as a layer over a TiO.sub.2 layer or in admixture with TiO.sub.2 on mica. X-ray investigation of these pigment shows that the TiO.sub.2, in spite of its tin content, is present in the anatase modification.
According to published German specification No. 2,214,545, pigments are preferred in which the metal oxide layer consists preponderantly of rutile TiO.sub.2 plus a small amount of tin oxide, with the concentration of the tin oxide being greater in the regions closest to the mica. The SnO.sub.2 content of the metal oxide layer is preferably 0.5 to 5 wt. % but in no case to more than 20 wt. %. However, it has been found that these pigments contain TiO.sub.2 which is not completely and not reproducibly in the rutile form but instead is very frequently present at least partially in the anatase form, although it is stated in published German patent specification No. 2,214,545, that the TiO.sub.2 was present "substantially in rutile form."
In German published specification No. 1,592,944, the addition of 1 to 50 wt. % of aluminum oxide during calcination of mica or of mica-containing pigments to bring about color stabilization of the mica, is described.
Bernhard et al., in U.S. Pat. No. 3,874,890, describe the precipitation of iron oxide on mica flakes coated with TiO.sub.2 and/or ZrO.sub.2 by precipitation of iron hydroxide from a ferrous salt solution and conversion to Fe.sub.2 O.sub.3. The nacreous pigment product has a golden-yellow hue.
Bernhard et al., U.S. Pat. No. 3,926,659, obtain nacreous pigments of various hues by formation of one only of .alpha.-, .beta.-, or .gamma.-FeOOH or magnetite in a continuous uniform layer on mica flakes by precipitation from an aqueous solution of an iron salt, in the presence of an oxidizing agent.
In U.S. Pat. No. 3,951,679, Bernhard et al. apply ferric hexacyanoferrate (II) (Berlin Blue) to mica flakes precoated with titanium dioxide or zirconium dioxide or hydrates thereof.
Esselborn and Bernhard, "Rutile-containing Lustrous Pigments," Ser. No. 616,843, filed Sept. 25, 1975, disclose the deposition of alternate layers, consisting at least of rutile TiO.sub.2, SnO.sub.2 and rutile, of metal oxides on mica flakes.
Lustrous pigments with a thin, uniform SnO.sub.2 layer on mica flakes and processes for the production of such pigments were thus hitherto unknown.
It has been found, in accordance with this invention, that suprisingly light-stable mica flake pigments with lustrous gloss are obtained by deposition of tin dioxide hydrate in a very smooth layer on mica flakes.